Case Sensitivity
https://www.w3.org/TR/2026/WD-selectors-4-20260122/#attribute-case

To match attribute values ASCII case-insensitively regardless of document language rules, the attribute selector may include the identifier i before the closing bracket (]). When this flag is present, UAs must match the attribute’s value ASCII case-insensitively (i.e. [a-z] and [A-Z] are considered equivalent).

Alternately, the attribute selector may include the identifier s before the closing bracket (]); in this case the UA must match the value case-sensitively, with “identical to” semantics [INFRA], regardless of document language rules.

The text about case insensitive (ASCII case fold) matching probably should have some sort of prominent health warning so that users understand this is only an ASCII case fold.